Fuel cells are considered as direct replacements for batteries in many portable devices and as power supplies for recharging batteries, power for silent watch and remotely-placed sensors, and for use in robotics and electric vehicles. A fuel cell can provide uninterrupted power so long as the fuel is continuously supplied. The process involves the conversion of chemical energy in the form of hydrogen and oxygen directly to electricity, heat and water.
Polymer electrolyte membrane fuel cell (PEMFC) technology has exceptional promise owing to light weight, high power, low operating temperature and fast start up. Hydrogen is oxidized at the anode to produce electricity and hydrogen ions that migrate through the polymer electrolyte membrane to the cathode. The hydrogen ions combine with oxygen that is reduced at the cathode to produce water. Although the preferred fuel is hydrogen, difficulties remain with its safe storage, transport and handling in an economical and light weight system. An alternative to hydrogen gas is to reform liquid fuels such as alcohols, gasoline and diesel fuel, or compressed gases such as butane and ammonia to produce hydrogen, yet this adds to system weight and complexity.
A direct methanol fuel cell (DMFC) catalyzes the oxidation of methanol at the anode catalyst in close proximity to the polymer electrolyte membrane (PEM) absent a separate reforming process to produce electricity and hydrogen ions. Protons that are produced as a result of methanol oxidation diffuse from the anode through the hydrated PEM to the cathode, where oxygen is reduced and is combined with the protons to form water. Incomplete methanol oxidation leads to methanol also permeating through the PEM to the cathode catalyst. This not only reduces fuel efficiency but also permits methanol to react at the cathode thereby reducing cell performance. The PEM must contain and be permeable to water for good proton conductivity. Conditions suitable for diffusion of protons and water often also allow for diffusion of methanol. A conductive membrane that is selective by being more permeable to water than to methanol increases fuel cell performance and fuel efficiency by keeping methanol on the anode side of the membrane.
A conventional DMFC PEM is a perfluorosulfonic acid ionomer sold under the trade name Nafion® 117 (DuPont). Although Nafion® is a fairly good proton conductor, cost and poor methanol barrier properties have affected the acceptance of this material. The ionic conductivity of Nafion® is due to the ionization of hydrated sulfonic acid groups that result in solvated protons. Conductivity is generally reported to be in the range of 0.08 to 0.1 Siemen (S) cm−1, but the extent of membrane hydration is a critical factor in determining conductivity. Conductivity increases with water content, but at temperatures exceeding 100° C. it decreases as water is lost. A DMFC typically operates at temperatures ranging from 60° C. to 80° C., with self-heating generated by methanol oxidation, and airflow at the cathode may tend to dehydrate the Nafion® membrane thereby decreasing proton conductivity. Thus, there exists a need for a membrane that adsorbs and maintains significant amounts of water to improve ionic conductivity and stabilize cell operating performance, while being less permeable to methanol than conventional membranes.